Conventionally, there is known a lighting device using light emitting diodes (LEDs) as a light source. In order to control the LED brightness, the conventional lighting device performs PWM dimming control in which a current flowing in the LED intermittently stops at a low frequency within a range from about 100 Hz to several kHz, or amplitude dimming control for changing an amplitude of the LED current. In the PWM dimming control, brightness of the LED is controlled by changing a time period (on duty) for supplying the LED with a current, and controlling an average value of an optical power (LED current). In the amplitude dimming control, brightness of the LED is controlled by changing a magnitude (amplitude) of the LED current, and controlling an average value of the optical power (LED current).
When the PWM dimming control is performed by using the PWM signal, it is preferable to set a frequency of the PWM signal to be equal to or greater than 100 Hz in order to suppress flickering of the LED. By setting the frequency of the PWM signal to be equal to or greater than 100 Hz, human eyes cannot notice the flickering under the LED illumination.
However, when the frequency of the PWM signal is set to be equal to or greater than 2 kHz, an on/off time interval is reduced in a region having a high illumination level. Accordingly, it becomes difficult to exactly control a switching device by using a pulse. Further, a noise occurs due to a transformer or the like. For that reason, when the PWM dimming control is performed, it is preferable to set the frequency of the PWM signal ranging from 100 Hz to 2 kHZ.
Further, there is disclosed an illumination apparatus capable of performing stable dimming control in a region having a high illumination level and suppressing a noise due to the transformer by combining the PWM dimming control and the amplitude dimming control (see, e.g., Japanese Patent Application Publication No. 2009-54425).
FIG. 7 shows waveform diagrams of the LED current supplied to the LED and the PWM signal in the PWM dimming control. As shown in FIG. 7, a light increasing period T11 and a light decreasing period T12 are alternately repeated, and, actually, the LED is turned on/off at the frequency of PWM signal. When the frequency of the PWM signal is set to be equal to or greater than 100 Hz, the flickering is seen on the average by the human eyes, which does not cause discomfort.
However, since a video camera captures an image at a constant shutter speed, e.g., 1/120 seconds or the like, flickering occurs on the images captured by the video camera under LED illumination. That is, even though the LED illumination does not cause discomfort to the human eyes, the human eyes can notice the flickering in the image captured by the video camera.